1. Field of the Invention
The present invention relates to an optical wireless transmission system for performing data communication using a light beam which is transmitted in an optical space, and more particularly to an optical wireless transmission system capable of reducing degradation of transmission characteristic caused by unnecessary reflection of a light beam.
2. Description of the Background Art
In recent years, optical wireless transmission technology for performing data communication by emitting a light beam into free space has been attracting attention, since, for example, the optical wireless transmission technology allows an information processing terminal or an AV equipment to be connected to another device in an office or at home without using a cable, and is expected to realize high speed communication utilizing broadband performance of a light. Further, as compared to a communication method for performing radio communication by using a wireless LAN or UWB, the optical wireless transmission is greatly advantageous in that a frequency to be used is not legally restricted. And, confidentiality and security are ensured since, for example, the optical wireless transmission enables enhancement of a directivity and exerts no influence on a neighboring wireless network.
Recently, an optical wireless transmission system capable of transmitting an increased volume of data at an increasingly enhanced speed has been required.
However, in order to perform data communication at an enhanced speed, it is necessary to provide a light source (light emitting element) and a light receiving element each of which enables high speed response and an enhanced performance. In particular, the light receiving element is required to have a reduced parasitic capacitance so as to represent an enhanced frequency response characteristic, and therefore it is necessary to reduce a light receiving area thereof. For example, the diameter of the light receiving element for 1.25 Gbps data communication is 200 μm and the diameter of the light receiving element for 10 Gbps data communication is 60 μm (as of February in 2006, according to Albis Optoelectronics AG)
However, in an optical wireless transmission system, reduction of the light receiving area of the light receiving element leads to reduction of total light receiving power, and therefore it is difficult to perform high speed transmission.
Therefore, in order to solve this problem, a multiplex transmission method is suggested in which parallel data communication is performed by using a plurality of light sources and a plurality of light receiving elements so as to enhance an entire transmission rate. See, for example, Japanese Laid-Open Patent Publication No. 8-237204 (Patent Document 1) (page 8, FIG. 1). This method uses a plurality of optical links so as to reduce a transmission rate for each optical link.
FIG. 16 is a diagram illustrating a structure of a conventional optical space connection device disclosed in Patent Document 1. Patent Document 1 discloses a technology for performing a signal transmission in a computer or a communication processing device, and this technology is also applicable to an optical wireless transmission system.
In FIG. 16, a plurality of electrical signals are inputted to an optical transmitter module 30, and the plurality of electrical signals are converted into optical signals, respectively, by an LD driver array 32 and a light emitting element array 33, and a collimator lens 34 converts the optical signals into light beams, respectively, having different angles from each other, thereby emitting the light beams into space. The optical receiver module 35 receives, through a collecting lens 39, the light beams such that the light receiving elements of the light receiving element array 38 collect the light beams so as to generate optical signals, respectively, in accordance with incidence angles of the light beams. The light receiving element array 38 converts the received optical signals into electrical signals, respectively, and outputs the electrical signals to an amplifier array 37. Thereafter, the amplifier array 37 amplifies the electrical signals, and outputs the amplified electrical signals from the optical receiver module 35.
In the conventional optical free-space connection device, positions of the light emitting elements aligned on the focal plane viewed from the collimator lens 34 represent emission angles of the respective light beams outputted through the collimator lens 34. Further, positions at which the light beams incident on the optical receiver module 35 are collected on a light receiving surface depend on the incidence angles of the light beams. That is, in the conventional optical free-space connection device, positions at which the elements of the light emitting element array 33 are aligned are represented as angular information representing emission angles of the light beams, and the angular information is transmitted after a process of the representation. Therefore, even in a case where a positional relationship between the optical transmitter module and the optical receiver module is changed, when the collecting lens 39 is allowed to collect the light beams, no influence is exerted on the positions at which the light beams are collected on the light receiving surface. Therefore, in the invention disclosed in Patent Document 1, a tolerance for changes of the positional relationship between modules is improved as compared to a device using a conventional lens array.
However, the structure shown in FIG. 16 is disadvantageous in that the numbers of light sources and light receiving elements are increased, and therefore mounting areas of the light sources and the light receiving elements are increased, thereby increasing the sizes of the collimator lens 34 and the collecting lens 39. Further, the light beams emitted from the light emitting element array 33 are incident on the collimator lens 34 so as to be perpendicular thereto. Therefore, a returned reflected light is incident on the light emitting element array 33, which leads to unstable operation of the light emitting element array 33. When the light beams are incident on the light receiving element array 38, a portion of the light beams may be reflected by the surfaces of the light receiving elements, and the reflected light is further reflected by a surface of the collecting lens 39 so as to be incident on the light receiving element array 38. The light having been repeatedly reflected results in delayed signals. Therefore, the higher a data rate is, the larger this problem is.
Thus, in the optical wireless transmission system, transmission quality is influenced by the reflected light which is returned from a reflection point in an optical transmission line, and the repeated reflections.
In order to prevent the influence of the reflected light, a technique of allowing a receiver to perform noise-cancellation is suggested. See, for example, Japanese Laid-Open Patent Publication No. 2002-111585 (Patent Document 2) (page 10, FIG. 1).
FIG. 17 is a diagram illustrating a structure of a conventional optical wireless transmission system disclosed in Patent Document 2.
As shown in FIG. 17, an optical transmitter 44 of a main unit 41 transmits light signals LPa and LPb each having a different light emission spectrum, from a LED 54 and a LED 55, respectively, so as to be in phase with each other, depending on a signal P to be transmitted. An optical receiver 46 of a sub-unit 42 subjects the light signals LPa and LPb to photoelectric conversion by using a PD 67, and an operational amplifier 68 extracts only the signal P. Further, a signal transmitter 45 of the sub-unit 42 differentially transmits light beams −LCb and LCa each having a different light emission spectrum, from a LED 61 and a LED 63, respectively, depending on a signal C to be transmitted. An optical receiver 43 of the main unit 41 subjects to photoelectric conversion the transmission light beams −LCb and LCa by using a PD 52 and a PD 53, in accordance with the light emission spectrum, and an operational amplifier 51 extracts only the signal C.
Accordingly, a signal transmitted from the PD 52 of the optical receiver 43 to an inverting input terminal of the operational amplifier 51 is a signal (N+(−C)+P) obtained by superimposing, on a signal (−C) obtained by subjecting to photoelectric conversion the transmission light beam (−LCb) transmitted by the optical transmitter 45, a noise component (N) generated by subjecting an ambient light LN to photoelectric conversion, and a noise component (P) obtained by subjecting to photoelectric conversion a light obtained by reflecting, by a reflector 47, the transmission light LPb transmitted from the optical transmitter 44. On the other hand, a signal transmitted from the PD 53 of the optical receiver 43 to a non-inverting input terminal of the operational amplifier 51 is a signal (N+C+P) obtained by superimposing, on a signal (+C) obtained by subjecting to photoelectric conversion the transmission light beam (LCa) transmitted by the optical transmitter 45, a noise component (N) generated by subjecting the ambient light LN to photoelectric conversion, and a noise component (P) obtained by subjecting to photoelectric conversion a light obtained by reflecting, by the reflector 47, the transmission light LPa transmitted from the optical transmitter 44.
The operational amplifier 51 subtracts an output signal (N+C+P) received from the PD 52 at the inverting input terminal thereof, from an output signal (N+(−C)+P) received from the PD 53 at the non-inverting input terminal. That is, the operational amplifier 51 performs a calculation (N−N+C−(−C)+P−P=2C).
That is, a result of the calculation indicates that a signal transmitted through the terminal 54 from the operational amplifier 51 to a signal processor following thereto corresponds to a signal (2C) obtained by completely eliminating the noise component (N) generated from the ambient light LN, and the noise component (P) generated from the light reflected by the reflector 47.
However, in the conventional optical wireless transmission system disclosed in Patent Document 2, it is necessary to position the LED 54 and the LED 55 of the optical transmitter 44 of the main unit 41 close to each other so as to allow complete accurate elimination of the noise component (N) and the noise component (P) from a signal to be outputted by the operational amplifier 51. If the LED 54 and the LED 55 are not positioned close to each other, the transmission light beams transmitted from the respective light sources are incident on the reflector under different conditions from each other (for example, an incidence angle and/or an incident power are different among the transmission light beams), whereby each of the transmission light beams has a different reflected light power. Further, the transmission light beams are transmitted through the reflector to the PD in optical paths having lengths different from each other, and therefore a magnitude and/or a phase of the noise component (P) generated from the reflected light which is incident on the PD 52 and the PD 53 of the optical receiver 43 of the main unit 41 is different among the transmission light beams. Therefore, the noise component is not completely eliminated through the calculation.
Further, in the conventional optical wireless transmission system disclosed in Patent Document 2, an output signal obtained by the sub-unit 42 is 2P corresponding to a signal obtained by doubling the output signal P obtained by one LED. However, when the LED 54 and the LED 55 are positioned close to each other so as to attain a purpose described above, emission intensity density of the light beam is increased, and therefore it is necessary to reduce an output power of each LED or reduce the emission intensity density of the light beam by using a diffuser or the like so as to satisfy eye-safe conditions, so that it is not easy to simply obtain an output signal of a high intensity.
As described above, the conventional optical wireless transmission system disclosed in Patent Document 2 is disadvantageous in that, while an increased number of components, such as an arithmetic circuit for noise cancellation and the increased number of light sources, that is, two light sources of different wavelengths, are provided as compared to the numbers of light sources and components of a typical optical wireless transmission system, a data transmission speed is not sufficiently enhanced.